Cleanup

[c-mac]

I cleaned up all of our code warnings, and then used rust-clippy to help us out with Rust code best practices. Some things we've commonly been doing non-idiomatically:

• Using match with only a single outcome (replaced with if-let: https://doc.rust-lang.org/rust-by-example/flow_control/if_let.html)
• Using thing.iter() instead of &thing
• !thing.is_empty() instead of thing.len() > 0